DT125R ARCHIVE

satelliteone · DTR Senior Member

P.Valve FAIL!.

craigbulmer · Guru

should i do mine????

just had the second rebore, doing 5k atm.

shall i get fiddling??

ouch, looks and sounds expensive.
is there any damage to the piston?

edit: the first pic wasnt loading, ouch!

engine at full power, for 15 miles, the only car you would beat at that speed is a g-whizz.
unlucky but you were asking for it haha

-- Edited by craigbulmer on Monday 15th of March 2010 05:54:02 PM

satelliteone · DTR Senior Member

hahaha reload the page chap.. sorted pix.. piston big time fooobar..

mind u, ive done a good few k miles on this set up..but never thrashed the living bollox off it.. for so long.

its easy to do,, just take the P.V valve out.. shave a 1mm of the back side of the pv. its alloy so it not hard...how much did you over bore your barrel

satelliteone · DTR Senior Member

on a standard bore 56.00mm.. the rings will be further away from the PV, so its ok just enuf for exspansion.

when u bore out, the piston/rings it gets wider and can foul the P.V, might work ok. for a while with out doing this, but when your piston temps go high, then ur going to have some trouble.

i wonder if the 170cc kit or even the 150cc kits take this into account. may be a modded P.V

Malc · DTR Senior Member

Are you sure? A 56.0 to 56.5 rebore adds 0.25mm onto the radius of the piston rings, which in barely a bawhair?

I very much doubt Yamaha would supply 0.5mm oversize pistons if there was even a remote chance the piston and/or ring would hit the powervalve! The TZR250s in YPM are virtually all bored to the max oversize and I've never heard of this happening to any YPM bike. These are racing bikes which all tend to runner much hotter than a DT on the road would (you need to bear in mind porting jobs, etc.)

Either way, that sucks. Is your cylinder serviceable, or has that took a beating too? Sorry for your bad luck...

craigbulmer · Guru

i think the bigger cc kits move the powervalve accordingly.

im not sure how you have managed this mate, you can safely rebore the cylinder on a dt 4 times according to yammy.. the engine temp must have been fecking high, how did you not heat seize it haha??

i dont have any motorways and id never run my bike like that lol, might mod it, wont be to bothered, seemed plenty of space between the cylinder wall and the powervalve edge lol

i think my cylinder must be 57 mm then, 2 rebores, if its 56 mm standard and two 0.5mm rebores.

cant wait til my bike is fully run in .. done 250 miles so far :P

satelliteone · DTR Senior Member

hi malc, i dont kno the set up on the tzr, head design n stuff, maybe the pv postions relative to the piston face is differnt, i dont know..like to see it. to compare to the dt barrel.

well, if i bored say to the max on the barrel 2.5mm halfed its 1.5mm would i trust the position of where it is at the mo. 1.5mm closer to the p.v. thats cold too.
its well close as is., why the p.v rotates backwards(clockwise) is beond me. i would of designed it to rotate away from the pison, shutting down the port.

its well too close for my like-ing, but i could be wrong here..then again evidence of the rings were in the PValve. and the way the piston looks.. deff contact. no detonation,

the bike was running really well too. the revs were up at 9-10k for quite a while, then i backed off catching a car up, then it went pop.

the barrel came off well, only a few score marks. the rings ended up in the exhaust.

any way, ive modded the backface of the p.v for my own peice of mind for now..

Malc · DTR Senior Member

Well, I've seen enough to know I don't want that happening to mine - any chance you can be more descriptive in where you've machined the powervalve please? and I'll get my dremel out

The TZR250 (1KT/2MA) barrel is remarkably similar to the TZR/DTR barrel, except there's two of them obviously.

New piston and rings and away you go again?

craigbulmer · Guru

hmmm, bummer then.

do you think possibly it clipped the port hole not the powervalve? with the heat expansion?

hmmmm, very odd though!

i swear there was plenty of distance!
but if theres not then i might try it to

not got a dremel, rough n then fine wet n dry should do

satelliteone · DTR Senior Member

craig... nah deff the rings its hit on the pv. i got indents in the PV where the corners of the rings tried bedding into it. Im not letting it do it again. If it hit the port, there would lots of damage to it, i have none. just the PV.

try to explain malc, if u take the cap off the side of the barrel for the PV.. oil bottle side. the undo the allen key, pull 1 half of the PV out. some how turn the PV to tick over pos, you can see the clearence of the PV to the rings.

you can see where about the places to take off some alloy.. I took the whole PV out, and stuck them toghter, and hit the backside with the dremmel. I use the sanding drums, not metal cutters.

the arc of the pv was lowered, u will see where when u look at it. U can always slide 1 half of the PV so see how much to take off.

wish i took pix before in installed it all now.. but as u can see in the first post, the last pic, the gap is wider.

dead easy really. u get the hang of it malc.

craigbulmer · Guru

thats really dodgy.

btw, i tried taking the powervalve out when i had it rebored, how do you get it out??? i took the alan key bolt out, but didnt give :S

RudiRulez · DTR Senior Member

ooh woh, thats not good, i went flat out from ace cafe to south end and the engine didnt skip a beat also i was traveling with my ate on a DTX and his was cool too, only problem i had was the petrol tank had gotten hot expanded causing the petrol cap to get stuck on lol, took us about 15mins to get off.

whats the standard bore size? everyone says 2mm oversize is maximum you can safely do on a DT, but i think youve just proved them wrong.

satelliteone · DTR Senior Member

ta rudi, was u at 10K revs all the way..lol. nah.. it was 0.5mm over sized. 56.5mm but yeah.. i might be right try 2.5mm over sized. 58.5mm how close that would be to the piston. lol

RudiRulez · DTR Senior Member

yeh im telling yah lol i had that throttle pinned back the whole way there, only time i let off was when i switched to my reserve tank, highest my speedo hit was 77mph, my other 2 mates took the motorway (we was on an A road) and just as our A road met up with the A road they had taken off the motorway we ended up right next to each other lol, whats the odds of that.

i cant see what any of what ive just typed has anything to do with your engine but......whatever.

would you mind getting a pic of the modification you made to the power valve so others (like myself eventually) can copy.

satelliteone · DTR Senior Member

okies will do that l8ter today for graig n your self. like u ive ragged the bike before. ive regularly gone up a duel carrage way from here to plymouth, but the bike not been flat out all the way. sits about 65-75mph.. thats around 30 miles one way. ragged in a couple of places. 77 Rudi?.. what revs in 6th?

it was different this time, as the engine was at 10K all the time. 80+ mph and in places off the clock. even when i dropped to 5th it was back up to 10K revs. i belive it was running red hot, highest limit its ever been, The temp gauge at the time was showng quater, so the water temp was spot on, but the piston temp must have been excessive.did`nt seize, so oil, mixture is spot on as with fueling. and a nice golden brown in the plug.

craig.. the long allan key slides out, a good set of grips and pull. this is the oil bottle side, end cap PV. Probs full of crud m8...

satelliteone · DTR Senior Member

okies here goes.

first cap off..

undo allan key

pull 1 half of the PV

undo the other side and pull the other half out..

now u have the PV out..

area for modding

this was the original PV that came out, the one in the barrel is a spare, which i modded. just for me to work some magic on as i have a spare barrel to fit this 1 in..

if u do this right, you will get a good.0.5mm to 1mm off, so plenty of clearence. DONT shave all the way around, just 3/4 or 1 inch. smooth it out. nothin harsh..

if u take 1mm off, that equal to a 2mm over bore on the piston. ideal.. any more than this your over doing it, unless ur over boring to 59.00mm..lol..

good luck!

-- Edited by satelliteone on Tuesday 16th of March 2010 10:52:42 AM

element · DTR Senior Member

I heard about a guy who had a rebore (max oversize) and didnt have the PV modded and the same thing happened but after only a few miles. If you send your barrel and pv to stan stephens they mod the pv anyway just some places dont or dont know?. I only ever heard of this happening once though.

else play in the PV barrel could cause this...

standard bore is 56.5mm so max overbore is 2mm to 58.5..

-- Edited by element on Tuesday 16th of March 2010 01:07:11 PM

-- Edited by element on Tuesday 16th of March 2010 01:08:46 PM

Peter T · DTR Newbie

Yes i had the same problem, my bore is 56.94 mm and the rings were hitting the powervalve which sounded like the small end was knackered!!!

craigbulmer · Guru

okay .... im at 57 mm atm (2 rebores? am i right or retarded?). I think i might just go and check .......................................

Malc · DTR Senior Member

Jees! Like busses here - three all at once...!

Thanks for the heads-up and good informative pics satellite - definitely doing it before I put it back together!

satelliteone · DTR Senior Member

hi pete.. ahh i see u have a bit of a problem too. I rekon anything over standard is a deff PV mod. Mine was on it limits at 56.5, just unlucky me thinks.

craig, ur lucky so far. very lucky. take the PV right out m8.. inspect it. I be very curious if u have any marks.

good luck malc.. peice of mind m8.. least u know it aint gona foul it esp when u kick the ass out of the engine.

RudiRulez · DTR Senior Member

nice guide sat, my dt pretty much tops at 9500revs.

ah yeh craig, for the sake of 30mins work you may aswell do this, i cant see it hindering performance and its nice to have it done for peace of mind.

-- Edited by RudiRulez on Tuesday 16th of March 2010 07:05:45 PM

craigbulmer · Guru

yeahhh :/

il do it, will have to order another exhaust gasket to make sure its al sealed proper

fray · Guru

Could this be why my bike is sounding like a bag of spanners after my top end rebuild? The piston they took out was 1mm oversize so my barrel is at least 57mm. Ive had PV probs since i got it back aswell!! They having it back on Thursday to strip it again as i wasnt happy.

Ill the mechanics to check the rings for signs of marks from hitting the PV.

satelliteone · DTR Senior Member

just done alan my m8`s top end, that was like a bag of spanners too. We are thinking the same. or a crap piston with loose rings..lol.. all sounds alike to me..lol

satelliteone · DTR Senior Member

i just had a thought. bit out of the box, but its viable.

i need to talk to stan stevens really on the bad points. but why not change the direction of the PV valve. Instead of rotating backwards, swap the cables around. it would rotate forwards. clearing the piston evey time.

all your doing is shuttin the port down.. as long as the PV levels to the port roof at full power, then rotates back, ! what ya thoughts.

RudiRulez · DTR Senior Member

deffinatly something to think about, yeh because switching the cables around will make it shut when its supposed to be open and viceversa, but then if you reallign the powervalve correctly (without seeing it i guess about 90 degrees ainti clockwise) then itll work just fine.

Deffinatly plausible, imo anyway, not sure what it would do to low down performance, maybe it sits so closely to the piston for a reason?

ben350 · Guru

Worth checking that they have put the cables to the right places for the YPVS - on my early set-up you have to go 1-2 and 2-1 which seems odd as 1-1 and 2-2 would make more sense!!

satelliteone · DTR Senior Member

that why i need to talk to a dedicated 2T tunner like stan stevens rudi. Iam not shure how it would effect the low down power, any adverse effect on anything.

sure would be hell of a lot better, as you wont need to mod the P.V. gona look into it for sure.

got me new piston to day, and fine tuned the PV with the dremmel.

just out of interest i put the old P.V back into the barrel and slid in the piston with rings. The feeder guages gap i got was 20 thou. 0.5mm from the rings. thats was with some chunks taken off it too.

heres the pic of the modded 1, getting 40 thou about 1mm.

lots better.

Malc · DTR Senior Member

satelliteone wrote:

all your doing is shuttin the port down.. as long as the PV levels to the port roof at full power, then rotates back, ! what ya thoughts.

No mate, won't work - what the powervalve changes is the port timing, or when the exhaust port actually opens with the downstroke of the piston.

When barrels were tuned pre-powervalve (RD125LC, RD350LC (God invented this bike, BTW)) tuning included opening the exhaust port up so it opened earlier. Dogs down low, but mental powerbands were had. Too much and the powerband was like a knife-edge, usually resulting in holed/siezed pistons cos you had to work so hard to keep the mother revving in the power.